1. Field of the Invention
This invention relates to rotational coupling devices such as brakes and clutches and, in particular, to a rotational coupling device having structure to compensate for wear on the braking surfaces of the device.
2. Discussion of Related Art
Rotational coupling devices such as clutches and brakes are used to control transfer of torque between rotational bodies. One type of conventional device is illustrated in U.S. Pat. Nos. 5,119,918, 5,285,882 and 5,971,121, the entire disclosures of which are incorporated herein by reference. This device includes a rotor that is coupled to an input shaft for rotation with the input shaft about a rotational axis. A field shell is also disposed about the input shaft on one side of the rotor and is fixed against rotation. The field shell defines radially spaced, axially extending inner and outer poles between which an electrical conductor is disposed, facing the rotor. A brake plate is coupled to the field shell and axially spaced from the field shell. The brake plate is disposed on a side of the rotor opposite the conductor. An armature coupled to an output member is disposed on the same side of the rotor as the brake plate and is disposed axially between the rotor and the brake plate. The armature is coupled to an output member by a plurality of leaf springs. Energizing the conductor produces a magnetic circuit in the field shell, rotor and armature that draws the armature into engagement with the rotor and couples the input shaft and output member together for rotation. Upon deenergization of the conductor, the leaf springs draw the armature out of engagement with the rotor and into engagement with the brake plate to brake the armature and output member. Permanent magnets coupled to the brake plate are also used to create another magnetic circuit between the brake plate, the field shell and the armature to assist the leaf springs in braking the armature and output member.
Repeated engagement of the armature with the rotor during clutch engagement and with the brake plate during braking causes wear on the engagement surfaces of the armature, the rotor, and the brake plate. Over time, this wear increases the air gap that exists between the armature and the rotor when the armature is engaged with the brake plate. The increasing air gap requires increased current to draw the armature into engagement with the rotor and engage the clutch. The current demand ultimately exceeds the service constraints of the device thereby reducing the useful life of the device.
The inventor herein has recognized a need for a rotational coupling device that will minimize and/or eliminate one or more of the above-identified deficiencies.